Liquid crystal displays (LCDs) have become an important part of information display in today's information society. As the demand grows, there is a substantial need to reduce the power consumption of backlit LCDs, while making them more affordable to consumers. A current trend is to design and manufacture thinner, less costly, and more power efficient LCDs. A major power-consuming part of an LCD is its backlight module. According to Kobayashi et al., backlight units for backlit LCDs account for up to 80% of the electric power consumption and 40% of the material costs of the LCDs [Ref. 1]. Current LCD backlights are inefficient; five or even six diffusers and other optical elements may be needed to build an LCD backlight for notebooks, computer screens and for other applications.
Most LCD devices need backlight units. According to Kobayashi et al, it is desirable that advanced backlight units substantially reduce their electric power consumption. Cost reduction would also encourage the early adoption of improved LCD technology.
FIGS. 1 & 2 show two established ways to arrange light sources 102 (e.g., LEDs) for LCD backlighting. In the edge backlighting (FIG. 1), the LED light is diffused into the light guide 104 and the light is reflected from either the top or bottom surface structure of the light guide to the surface. In the direct-view backlighting (FIG. 2), light sources 102 with diffuser(s) 106 on top of them are placed directly behind the display. The direct-backlighting approach (FIG. 2) usually requires more electric power as more LEDs are usually needed to achieve the same display brightness uniformity [Ref. 2].
Referring to FIG. 3, current LCD backlights usually consist of 6-7 components: a reflector 302, light sources (exemplarily LEDs) 304, a light guide 306, two or three diffusers 308, one or two brightness enhancement films (BEFs) 310. The LCD 312 uses polarizers 314 to control the passage of light into and out of the LCD. A conventional backlighting solution works in the following way: light from an LED 304 is coupled into the dot-printed 316 light-guide plate (either flat or wedged). The wedged shape is often preferred as it tends to increase the light throughput efficiency by reducing end-face light loss [Ref. 3]. The light rays are scattered randomly when, they encounter a printed dot. Brightness enhancing films 310, seen as crossed micro-prismatic sheets, are used to manage the angle of light upwards thus enhancing brightness in the direction normal to the viewing plane, as in Refs. 4 & 5, and diffusers 308 are used to smooth out the spotted appearance caused by the dotted plate and by the prismatic structures. Thus, in a conventional approach, six to seven films and other components are needed to homogenize and evenly distribute light upwards from LEDs.
Films with linear structure interfere with the pixel structure of the LCD. The superposition of two spatial frequencies forms a series of observable fringes. This undesired moiré pattern substantially deteriorates the visual appearance of the display. To reduce moiré effects, at least one top diffuser must be placed between the LCD and micro-prismatic BEF film. The top diffuser(s) can be configured to decouple the interference of LCD and micro-prismatic films. But these top diffusers introduce an undesirable additional light loss (and, therefore, power efficiency loss) due to reflection and scattering. In general, each additional component introduces some extra light loss (2-4%) at each surface-air interface.
A single-component backlight unit is needed to reduce both the light losses on interfaces between the backlight components and the cost of the backlight unit (the number of components used). An early solution for such a system, i.e., for a substrate-guided diffuser-LCD backlight was proposed by Kaiser Optical Systems [Ref. 6]. Unfortunately, this solution cannot be reduced to practice as was experimentally shown in Ref. 7 because the laser light introduced into the light guide at some divergent (or convergent) cone angle will generate speckle, and presence of this speckle field is an obstacle in providing homogeneous speckle-free illumination for an LCD.
An unsuccessful multi-grating approach suggested by Samsung [Ref. 8] lacked a provision to control the diffusers' angular distribution. Another research attempt to provide an LCD backlight with a reduced number of components was made by Kuraray Co. [Ref. 9]. But the authors acknowledged that the brightness (luminance) of the backlight was not sufficiently increased; also, an additional element was needed to compensate for a too thin light-guiding plate.
Hitachi Chemical Co. Ltd. proposed a backlight device with improved efficiency using a surface relief hologram [Ref. 10]. However, a developed backlight device requires an additional light-guide plate with V-shaped groves, thus reducing the over-all light throughput efficiency. An LCD backlight with a single reflective holographic grating deposited on the lower side of the light guide wedge to replace a conventional reflector was described in Ref. 11. However, the low light throughput efficiency, substantial color dispersion of the single grating, and brightness non-uniformity of the developed backlight over the entire visible spectrum range proved to be serious issues that limit the effective use of such a system for LCD backlighting.
Achieving efficient light coupling from the backlight to the LCD can be brought about by using, e.g., linearly polarized light-emitting guide [Ref. 12]. While rather high-polarized contrasts can be achieved for out-coupled light, it is rather hard to efficiently recycle the trapped light with the orthogonal polarization.
Various approaches, e.g., using diffractive structures imprinted on transparent light-guide plates were tried [Ref. 3, Ref. 13]; however, it has not been possible to provide uniform illumination across the light-guide surface, and presence of undesired bright lines formed by diffracting images of the LEDs were quite visible. A cascade of a thick grating and a thin diffuser was shown to scatter radiation efficiently and uniformly over a wide angle [Ref. 14]; however such a system still requires two separate elements for the LCD backlight.
On the other hand, substrate-guided holography for visual applications is an established technology [Ref. 15, Ref. 16, Ref. 17, Ref. 18]. Approaches developed for visual applications can be successfully applied for non-imaging applications, such as, e.g. LCD backlighting and LED lighting. Combining substrate-guided holography with methods of fabricating light-control diffusers (e.g., holographic diffusers) is needed. Holographic diffusers are known, www.luminitco.com, to give a high light output gain in a given direction provided that either collimated or divergent light input is made at either normal or close to normal angle with respect to the diffuser surface. The need to have incident light at an angle close to normal with respect to the diffuser surface has prevented widespread use of holographic diffusers in the LCD backlighting industry thus far, because one or two additional optical films (e.g., DBEF films manufactured by 3M) are needed to collimate the light from LEDs (or other types of light sources) towards the holographic diffuser.
A conventional way to extract light from a light-guiding plate for an LCD backlight is to provide a dot-pattern on one side of the light-guide plate. The dots in the pattern can be distributed either homogeneously or non-homogeneously, according to an optimization design procedure based, e.g., on a molecular dynamics computational algorithm [Ref. 19]. Such design procedures are extremely computationally intense. In most cases, computational optimization is never complete because of the prohibitively long computational time needed. After a few initial optimization runs, a trial backlight mold is made, its light-extracting parameters are experimentally measured, then, based on experimental results, a few more optimization runs are made, and another trial mold is made, etc., until a satisfactory light-extracting performance is achieved. This conventional approach is very expensive, especially when a non-homogeneously distributed spatial dot pattern is required. Replacing it with a holographic design approach provides a way to substantially reduce the cost of the LCD backlight design.
In addition to LCD backlighting, an efficient substrate-guided diffuser is strongly needed for LED lighting. Municipalities and commercial enterprises look to LED-based lighting as a way to make major reductions in energy usage [Ref. 20]. The term ‘LED lighting technology’ currently implies design, manufacture, and/or integration of illumination/lighting elements for architectural, street, advertisement, signage, and other lighting based on LED light sources. An existing LED lighting technology consists of one or more LEDs, and LED electronic driver, and light-shaping elements (diffusers, plastic or glass lenses, plastic or glass sheets, light reflectors and/or concentrators, or any combination of these elements) [Ref. 20, Ref. 21]. Thus, a thin, substrate-guided light-shaping diffuser is needed for LED lighting technology.